Video: Figuring Out the Rule of a Quadratic Function given Its Graph

Video Transcript

Which of the following graphs
represents 𝑓 of 𝑥 equals 𝑥 squared plus one?

Almost immediately, I notice that
the leading coefficient here is positive. What do I mean by that? This 𝑥 squared is a positive
number. You can imagine it as one times 𝑥
squared, so its coefficient is one, positive one.

Since our 𝑥 squared value is
positive, our parabola will open upward. This means that both (b) and (d)
are not possible options for graphs of this function. But now we need an additional point
to help us figure out if (a) is our graph or (c) is our graph.

To do that, I want to choose a
point that we can see on our graph. Let’s plug in zero for 𝑥. If we plug in zero for 𝑥, we get
zero 𝑥 squared plus one. This function at 𝑥 equals zero
equals one. The point zero, one is on this
parabola, so let’s graph it. There’s point zero, one on graph
(a), and here’s point zero, one on graph (c). Zero, one is a point on both (a)
and (c), so we’ll need to choose an additional value to check.

Let’s choose another number. Let’s check the value of the
function when 𝑥 equals one. That would be equal to one squared
plus one.

The value of this function when 𝑥
equals one is two. We found an additional point of
one, two. Let’s graph that one. Here’s the point one, two on graph
(a), and here’s the point one, two on graph (c). The point one, two doesn’t fall on
the line in graph (c); we can eliminate it. Graph (a) represents the function
𝑓 of 𝑥 equals 𝑥 squared plus one.